The experiments in this proposal are designed to identify and characterize T cell costimulatory pathways that prevent the induction of peripheral T cell anergy or cell death as a consequence of T cell antigen receptor (TCR) engagement. It is believed that under most circumstances TCR-mediated signal transduction serves primarily to activate a T cell to a state where the cell must make a further decision to proliferate and acquire effector function; be rendered anergic; or induced to die by apoptosis. Which fate the cell chooses is determined by accessory or costimulatory signals received by the cell either at the same time as TCR engagement or shortly thereafter. One candidate costimulatory pathway has been defined which is mediated by the CD28 receptor on the T cell interacting with its ligand B7 on the antigen-presenting cell (APC). To examine the role of the B7/CD28 activation pathway in the regulation of peripheral T cell responses, CD28-deficient mice have been produced. The ability to induce peripheral tolerance in response to various forms of antigenic challenge will be examined using these CD28-deficient mice in order to determine the significance of CD28 in various forms of immune response. By studying the responses of CD28-deficient T cells to antigen presentation by individual types of APCs, we hope to identify additional costimulatory pathways and determine their role in inducing/preventing peripheral tolerance. As part of these studies, the molecular mechanisms by which costimulatory signals prevent antigen-receptor desensitization and promote IL-2 production will be investigated. Other mechanisms by which costimulatory pathways may affect a peripheral immune response will also be explored. For example, we have found that CD28 costimulation can enhance survival of cells activated through the TCR. This effect was independent of the ability of CD28 costimulation to enhance IL-2 production. As a result of these observations, a novel CD28-inducible gene, bci-x, has been identified which is closely related to bcl-2. The ability of different costimulators to induce bcl-x expression and enhance cell survival will also be examined. As a result of these studies, we hope to define specific strategies to regulate peripheral T cell tolerance through the modulation of T cell costimulatory signals.